1. Field of the Invention
The present invention relates to an internal-termination type polarity-reversal battery-feed circuit for supplying a speech current or supervision current to a subscriber line or trunk line of an exchange.
The battery-feed circuit according to the present invention not only functions to supply speech current for a subscriber or trunk line, transmit a voice signal to the line, and terminate the voice signal from the line, but also to send a polarity-reversal line signal.
2. Description of the Related Art
An exchange is equipped with a battery-feed circuit for supplying a speech current to telephone sets. The main functions of this battery-feed circuit are as follows:
(i) Supply of the speech current and/or the supervision current; PA1 (ii) Transmission of voice signals to the speech line and the reception thereof from the speech line; PA1 (iii) Termination of the speech line for the received voice signal; PA1 (iv) Transmission of the polarity-reversal signal, etc.
This prior art battery-feed circuit requires separate circuits for each of these functions, and therefore requires many components, which enlarges the battery-feed circuit, complicates it, and raises its cost. Further, one of its components, an electronic inductor circuit, has a high impedance against common mode signals (longitudinal signals), and therefore cannot terminate an undesirable common mode signal in the speech line to ground using a low resistance. Further, when noiseless polarity reversal is required, and LC filter circuit is needed, which has the disadvantages of increased weight and poor packing density. The capacitance and inductance elements in the LC filter circuit make it difficult to form the battery-feed circuit as an integrated circuit.